The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the speed of an engine in accordance with driving conditions of the vehicle.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of an engine is transmitted to driving wheels of the vehicle through a clutch and the transmission. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When an accelerator pedal is depressed to increase engine speed, the speed of the vehicle increases, keeping the transmission ratio at the maximum value. A line 1.sub.1 of FIG. 5 shows the increase of engine and vehicle speed. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to reduce (to upshift). After that, the transmission ratio is automatically and continuously reduced at a changing speed and engine speed and vehicle speed change along a line 1.sub.2 by operation of the driver of the vehicle.
During the upshifting, the engine speed is substantially constant or slightly increases. Generally, the engine speed, at which the transmission ratio starts to upshift, is set to a high speed, corresponding to a wide open throttle. Accordingly, the engine makes a loud sound in a range from a low vehicle speed to a high vehicle speed. In order to reduce the engine sound, if the engine speed is decreased to a line l.sub.3, the acceleration characteristic at a high vehicle speed range decreases and the maximum vehicle speed at a maximum power is reduced.